1. Field of the Invention
The present invention generally relates to voice and data communication systems. More particularly, this invention relates to wireless communication systems including formatting of data to be transmitted over an air interface at a high data rate.
2. Description of the Related Technology
The T-carrier system provides high rate digital transmission of data to customer premises. Originally, this system was designed for wired networks in order to enhance the quality of calls and better utilize the cable facilities. In particular, the T-carrier technology allowed telecommunication companies to increase the call carrying capacity while taking the advantage of unused transmission capacity of their existing wire pair facilities, as well as improving the transmission quality.
A first generation of T-carrier systems, called T1 (or Digital Signal Level 1, DS1), is a full duplex all-digital service. The digital stream is capable of carrying standard 64-Kb/s channels in which 24 channels are multiplexed to create an aggregate of 1.536 Mb/s. Time-Division Multiplexing (TDM) allows a channel to use one of the 24 timeslots. More specifically, the 24 channels are time-division multiplexed into a frame to be carried along the line. Each frame contains one sample of 8 bits from each of the channels. Added to this is a framing bit. This structure results in a frame of 193 bits. There are 8000 frames per second (due to the PCM on each channel), therefore a frame is 125 microseconds long. Adding the 8 Kb/s overhead (due to framing) to 1.536 Mb/s, yields an aggregate of 1.544 Mb/s. T1 usually employs AMI (Alternate Mark Inversion) coding in order to reduce the required bandwidth of 1.5 MHz by a factor of two. The transmission is byte-synchronous, i.e., timing for each channel is derived from the pulses that appear within the samples (8 bits in each sample). This timing keeps every thing in sequence. Although, T1 is generically 24 channels of 64 Kb/s data plus 8 Kb/s of overhead (sometimes called channelized service), the multiplexing equipment can be configured in a number of ways. For example, T1 can be used for a single channel of 1.536 Mb/s, two high-speed data channels at 384 Kb/s each, and a video channel at 768 Kb/s. In short, the T1 service does not have to be channelized into 24 timeslots. It can be any usable data stream required. Although T1 systems are generally treated as four-wire circuits, they can also support any other suitable medium such as fiber optics, digital microwave links, coax, etc. When the other media forms are used, the T-carrier is suitably taken from the transmission mode and converted back to the appropriate interface.
A T2 (DS2) service has a concept similar to T1, but with a data rate that is four times higher and some extra framing bits. This results in an aggregate of 6.312 Mb/s. Similarly a T3 digital link is composed of a serial combination of seven T2 links (and the required extra framing bits), resulting in a data rate of 44.736 Mb/s. The T3 service is utilized in high capacity services.
The E carrier services are the European equivalents of the T-carrier. Table 1 contrasts different T and E carrier systems in terms of their TDMA structure and data rates.
TABLE 1T/E carrier systemsT-CarrierTDMA Structurebits/125 μsData rateDS08 bits/125 μsec 8 b/125 μs64Kb/sT1, DS124 DS0 + 1 Framing bit =193 b/125 μs1.544Mb/s(24 × 8 + 1) bits/125 μsecT2, DS24 DS1 + 17 Framing bits =789 b/125 μs6.312Mb/s(4 × 193 + 17) bits/125 μsecT3, DS37 DS2 + 69 Framing bits =5592 b/125 μs 44.736Mb/s(7 × 789 + 69) bits/125 μsecE-CarrierTDMA StructureData RateE064Kb/sE130 E0 + Framing Bits2.048Mb/sE24 E1 + Framing Bits8.448Mb/sE34 E2 + Framing bits34.368Mb/s
Due to the considerable cost of wiring, line amplifiers etc., there has been an increasing need for wireless implementation of T/E-carrier technology. Initially, infrared laser-based T1/T2 systems were developed. The major concern was the fact that no licensing requirements were necessary, so that the system could be put into action as soon as the acquisition was made. In addition, the cost of implementation required no major tower, power equipment, cable entrances or other construction needs. Typical applications of such cordless T1/T2 links are digital PBX to PBX connection (using a quad T1) and video conferencing using a channel capacity of 6 Mb/s or four 1.544 Mb/s at compressed video standards. However the infrared laser technology has a number of disadvantages, such as limited range (up to 1.5 miles), concern over the use of a laser in an office environment, atmospheric disturbances, etc.
Recently, there has been a growing interest in implementing T-carrier systems using wireless microwave (point-to-point) radio links (e.g., Radio in the Local Loop or RLL). Such applications have been facilitated by use of spread spectrum technology and the recent release of the unlicensed ISM (Industrial, Scientific and Medical) bands by FCC. The latter overcomes the frequency coordination and licensing problems associated with microwave communications. These bands, which are at 900 MHz (902–928 MHz), 2.4 GHz (2400–2483.5 MHz) and 5.7 G Hz (5725–5850 MHz), are defined under section 245 of the part 15 of the FCC regulations.
There is an urgent need for wireless T/E technologies, especially for mountainous region extension, urban links between separate facilities, over-water extensions, site interconnections of cellular networks, building-to-building LAN extensions, PBX, FAX and data extensions, and community networks.
One limitation of conventional T/E carrier systems for synchronization between the transmitter and the receiver is that they use framing overhead bits of successive frames. In other words, the frames are sent sequentially to ensure synchronization in conventional T/E carrier systems. Furthermore, the initial T/E systems were used to reduce the number of voice frequency cable pairs needed for interconnecting telephone offices. Many of these links were short and analog cable systems had not proved economical. More importantly, there were technical complications associated with these initial T/E systems as they became more widely deployed. One major complication is that the speech coding used was inadequate for providing proper transmission quality to create long-distance circuits. To prevent this problem, the number of T systems in series had to be limited to three, which substantially complicated network provisioning and circuit planning.
In view of the foregoing, there is a need in the industry for a new method of implementing T/E systems which extends the coverage area in a wireless communication environment without the disadvantages of conventional methods. The new method and system should enable compensation for transmitting and receiving frequency variations, synchronization at the receiver and provision of a virtual signaling channel. This method and system should expand coverage areas while maintaining minimal channel inter-cell interference or congestion. Furthermore, such a system should be easy to install and maintain.